1. Field of the Invention
The present invention relates to a defect detecting apparatus for insulators, and in particular to an apparatus for detecting a defect such as a pinhole or the like on an insulator of a spark plug for an internal combustion engines.
2. Description of the Prior Art
A conventional defect detecting method for insulators has detected a pinhole in an insulator since such will cause misfiring of a sparkplug for an internal combustion engines.
This defect detecting method is based on the application of a voltage between the center electrode and the housing of a sparkplug generating a spark discharge between the center electrode and an earth electrode normally if no defect is present in the insulator of the spark plug. However, if there is a defect, then spark discharge between the center electrode and the housing will occur while no spark discharge occurs between the center electrode and the earth electrode. Therefore, such defect detection of the insulator is very important.
Accordingly, the applicants have proposed the following method of detecting a defect in an insulator as a defect detecting method for an insulator, based on the path of spark discharge differing depending upon whether a defect in the insulator present or not.
A specific form of the above-mentioned defect detecting method will be explained hereinbelow with reference to FIG. 7.
In order to detect a pinhole as one of detects in the insulator of spark plug, a first electrode 3 is inserted in a hollow section 1a of an insulator 1, a second electrode 5 is set at a position outside of the insulator where electric discharge is possible, as shown in FIG. 7, and a potential difference is generated between the electrodes 3 and 5 by a high voltage generating device 10 as an electric discharge means so as to cause electric discharge between the first electrode 3 and the second electrode 5.
This arrangement allow the path of the produced spark discharge to differ between a satisfactory insulator in which no pinhole is present and a defective insulator in which a pinhole is present, as shown in FIGS. 8A and 8B. That is, in the satisfactory insulator in which no pinhole is present, discharge follows an electric discharge path "a" which passes through the front end surface 1b of the insulator 1 as shown in FIG. 8A, but in the detective insulator in which a pinhole is present, discharge passes an electric discharge path "b" which passes through the pinhole as a defect in the insulator 1 as shown in FIG. 8B. Accordingly, the difference between the spark discharge paths "a" and "b" is visually checked by a manual technique (human eyes) so as to determine whether the insulator is defective.
By using the above-mentioned method, a defect in a simple insulator can be detected, and accordingly, it is sufficient to discard only insulators in which a defect is detected. Thereby the checking of insulators have been rationalized. However, in the above-mentioned detecting method, the following drawbacks have been noted. First, since the difference between the spark discharge paths is manually checked, human error may cause variations in the rate of detection of defects. Therefore, the reliability of the checking becomes dependent on the reliability of the checker. Further since the man-hours for the checking are large, the cost of total inspection becomes high.
Accordingly, the present invention is devised in view of the above-mentioned drawbacks, and it is an object of the invention to allow automatic checking whether an insulator is satisfactory instead of using a visual inspection in the above-mentioned checking steps.